In recent years, a reduction in CO2 emissions from automotive exhaust gas has been strongly desired in view of global warming. It is effective to increase the travel distance with a limited amount of fuel (i.e., improve the fuel efficiency of automobiles) in order to reduce CO2 emissions from automobiles. It is known that an improvement in hardware (internal combustion engine) and an improvement in engine oil (i.e., lubricant) significantly contribute to an improvement in fuel efficiency of automobiles.
For example, an engine oil for which the friction coefficient in a boundary lubrication region is reduced by adding a friction modifier such as an organomolybdenum compound is known as an engine oil that improves fuel efficiency (see Patent Literature 1 and 2, for example).
It is also effective to improve the viscosity characteristics of an engine oil in order to improve fuel-efficiency. For example, it is known that an engine oil composition to which a specific viscosity index improver is added either alone or in combination improves fuel efficiency (see Patent Literature 3 to 5, for example).